1. Field of the Invention
The present invention relates to a thin film transistor and a method of fabricating the same and, more particularly, to a thin film transistor having excellent characteristics and a method of fabricating the same in which a dual capping layer is formed of an oxide layer that a metal catalyst is difficult to diffuse and a nitride layer that a metal catalyst is easy to diffuse, a metal catalyst layer is formed on the dual capping layer, and then a crystallization process is performed to form a polysilicon layer having a large-size grain because the metal catalyst does not easily pass through the oxide layer and so a small amount of metal catalyst contributes a crystallization, and a semiconductor layer is formed of the polysilicon layer.
2. Description of the Related Art
In thin film transistor (“TFT”) used in a display device, a semiconductor layer is formed such that an amorphous silicon layer is deposited on a transparent substrate made of a glass or a quartz and the amorphous silicon layer is subjected to a dehydrogenation treatment and then is crystallized.
At this time, the semiconductor layer which constitutes a source, a drain and a channel area is formed by depositing an amorphous silicon layer on a transparent substrate made of a material such as a glass using a chemical vapor deposition (“CVD”) technique. The silicon layer deposited directly on the substrate using a CVD technique is an amorphous silicon layer which contains a 12% of hydrogen and thus has a low electron mobility, and when the amorphous silicon layer having such a low electron mobility is heat-treated and crystallized into a silicon layer of crystalloid structure having a high electron mobility, the silicon layer may be damaged since hydrogen contained therein may burst. In order to prevent a burst of hydrogen which may occur during crystallization, a dehydrogenation process is carried out. The dehydrogenation process is performed such that a heat-treatment is performed in the furnace at a temperature of more than about 400° C. for tens of minutes to tens of hours. Then, the dehydrogenated amorphous silicon layer is subject to a crystallization process.
The crystallization technique which crystallizes an amorphous silicon layer to form a poly silicon layer includes a solid phase crystallization technique, an excimer laser crystallization technique, a metal induced crystallization (MIC) technique, and a metal induced lateral crystallization (MILC) technique. The solid phase crystallization technique is one which heat-treats and crystallizes an amorphous silicon layer for several hours to tens of hours at a temperature of less than about 700° C. which is a temperature that may transform a glass which forms a substrate of a display device on which a TFT is formed. The excimer laser crystallization process is one which scans an excimer laser to an amorphous silicon layer to be heated and crystallized at a high temperature for a very short time.
However, the solid phase crystallization technique has disadvantages in that a relatively lengthy processing time is required and a substrate is exposed to a high temperature for a long time and thus may be easy to transform. The excimer laser crystallization technique has also disadvantages in that a high price laser device is needed and also an extrusion may occur on a crystallized surface so that an interface characteristic between a semiconductor layer and a gate insulating layer is bad. The MIC or MILC technique has disadvantages in that a large amount of metal catalyst remains on the polysilicon layer to thereby increase a leakage current of the semiconductor layer of the TFT.